a) Field of the Invention:
This invention relates to a gradient index measuring apparatus utilizing the interference of light waves for measuring the gradient index of a sample and, more particularly, to an apparatus for measuring the asymmetry of the gradient index in a gradient index lens.
b) Description of the prior art:
In the past, where an interferometer is used to measure the gradient index of a sample, a method has been adopted that, as set forth in U.S. Pat. No. 4,842,408, the contour of interference fringes caused by the gradient index of the sample is measured and, based on the resultant information of phase distribution, a reduced equation is used to calculate the gradient index. The gradient index measured by such a method involves a conversion error and consequently, it is necessary to take account of the error as a tolerance in designing an optical system using a gradient index optical element on the basis of the measured value. In the prior art, when an optical system is designed in which an optical element having the gradient index radically, namely, concentrically in particular, is employed as a lens, consideration should be taken of, in addition of the tolerance, the influence of the asymmetry of the gradient index caused about the center of the lens on its properties. However, the method and apparatus for measuring the asymmetry of the gradient index have been unavailable in the past.
For inhomogeneity, an apparatus is known for measuring the inhomogeneity of a refractive index in homogeneous glass (strictly speaking, it also has inhomogeneity), for instance. This apparatus is such that homogeneous glass, both surfaces of which are ground into parallel plane surfaces, is placed in the interferometer, the parallel plane surfaces are normally irradiated with light, and the interference fringes of wave fronts of the transmitted light are observed, thereby measuring the inhomogeneity [J. Schwider et al., APPLIED OPTICS, Vol. 24, No. 18, pp. 3059-3061 (1985)]. According to the apparatus, the inhomogeneity of the sample can be measured in which the refractive index is not distributed homogenously. However, the interference fringes caused by the wave fronts transmitted through the homogenous glass merely represent a state of the deflection of light attributable to the inhomogeneity, whereas those appearing in measuring the sample with the gradient index include the information on the gradient index, such as a phase shift attributable to the thickness of the sample, as well as on the inhomogeneity of the refractive index. Hence, such interference fringes cannot bring about only the information on the inhomogeneity of the refractive index, that is, on the asymmetry of the gradient index. Since the proper method and apparatus for measuring the asymmetry of the gradient index have been unavailable, the quality of the sample has been unable to be determined in the measurement of the gradient index or the design.